The formation and consumption of nanometer scale precursor particles during the hydrothermal synthesis of
Si-TPA-MFI from a clear solution has been studied in situ using a combination of X-ray scattering techniques
and with electron microscpy. The combination of wide-, small-, and ultra-small-angle X-ray scattering allowed
us to obtain information on a continuous range of length scales spanning over four orders of magnitude
(0.17−6000 nm), covering all particle populations present during the complete course of the crystallization
process. The use of high-brilliance synchrotron radiation allows us to perform time-resolved experiments.
Two types of precursor particles were observed: 2.8 nm sized primary units and aggregates (≈10 nm). Variation
of the alkalinity of the synthesis mixture revealed a strong correlation between the concentration of the
aggregates and the rate of the crystal nucleation. The presence of the 2.8 nm sized primary units appears to
be independent on the alkalinity. The addition of seed crystals to a synthesis mixture that does not show
spontaneous nucleation (no aggregates observed) resulted in normal crystal growth. The size distribution of
the growing crystals could be followed in situ by fitting calculated scattering patterns to experimental curves
and showed good agreement with electron microscopy results. The apparent activation energy for crystal
growth is determined to be 83 kJ/mol by following in situ the crystal growth process at various reaction
temperatures. These data show that the formation of aggregates of primary units is an essential step in the
nucleation process and suggest that the crystal growth step is the reaction-controlled inclusion of the 2.8 nm
sized primary units at the crystal surface.
Aggregates of fractal dimension were found in the intermediate gel phases that organize prior to nucleation and crystallization (shown right) of silicalite from a homogeneous reaction mixture. Small‐ and wide‐angle X‐ray scattering studies prove that for zeolites nucleation may be homogeneous or heterogeneous.
The formation of precursors and the growth and aggregation of silicalite-1 crystals using tetrapropylammonium as a template (Si-TPA-MFI) has been studied in situ using X-ray scattering techniques. Simultaneous smallangle and wide-angle X-ray scattering (respectively SAXS and WAXS) experiments using synchrotron light showed that the formation of amorphous colloidal aggregates in water clear synthesis mixtures was dependent on the alkalinity of the solutions. In situ time-resolved ultra-small-angle X-ray scattering (USAXS) showed the form factor oscillations of the growing crystals. Fitting of the USAXS patterns to the scattering pattern of spherical particles having a normal particle size distribution showed a linear growth of the average crystal diameter, which was approximately the same for both alkalinities studied. The final size of the crystals was highest for the synthesis mixture having the highest alkalinity, which can be explained in terms of number of viable nuclei formed. At the end of the linear growth the crystals form aggregates corresponding with a diffusion limited aggregation process (mass fractal dimension of 1.8).
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